Ultrafast dynamics of excess electrons in a molten salt: Femtosecond investigation of K–KCl melts

Abstract

Pump–probe studies of localized excess electrons in liquid KCl have been performed at probe wavelengths between 600 and 1530 nm at elevated temperatures. After photoexcitation with 50 fs pulse centred at 800 nm a transient bleach is observed at wavelengths between 600 and 1245 nm while at wavelengths longer than 1300 nm only a transient absorption is observed. The transient bleach recovery can be assigned to the lifetime of strongly localized electrons (F-centres) and represents ground state dynamics with a time constant of (200 ± 50) fs, in reasonable agreement with theoretical calculations for polaron-like states in liquid K–KCl. The transient absorption at longer wavelengths, that decays with the same time constant, might be due to weakly localized electrons. Consequently, the ionic motion of the alkali halide melt determines the dynamics in this system. These investigations represent the first direct ultrafast study with fs time resolution of localized electrons in the molten K–KCl sytems.